1. Field of the Invention
The present invention relates to a digital directional relay that determines a direction of a fault using a variation in amount of electricity of unbalanced components such as a variation in negative-phase-sequence components and a variation in zero-phase-sequence components.
2. Description of the Related Art
Directional relays for determining the direction of a fault occurring in a power system using an amount of electricity of negative-phase-sequence components or that of electricity of zero-phase-sequence components are widely known by, for example, Electric Technology Research Association Report, Volume 37-1, pages 54–55.
Assume that the sample values of voltage and current of the power system, which are obtained at regular sampling intervals, are Vm and Im, respectively in order to implement a digital negative-phase-sequence directional relay. The negative-phase-sequence components of the voltage and current are computed by the following equations (1) and (2):3V2m=Vam+Vb(m−8)+Vc(m−4)  (1)3I2m=Iam+Ib(m−8)+Ic(m−4)  (2)wherein m is a sampling point in time, subscript 2 is a negative-phase-sequence component, a, b and c are amounts of electricity of A-, B- and C-phases, and m−α is an amount of electricity that is generated α-sampling before m. The sampling interval corresponds to an electrical angle of 30 degrees.
Since there are no negative-phase-sequence components when a power system is kept in three-phase equilibrium, the values of respective terms of the equations (1) and (2) are zero (0). If an unbalanced fault occurs in the power system, the negative-phase-sequence impedance Z2 for a system protection relay satisfies the following equation: V2m=Z2×I2m. Since most of the negative-phase-sequence impedance Z2 is generally reactance components, V2m and I2m are out of phase with each other by almost 90 degrees. The matter as to which phase of voltage and current advances depends upon whether the fault occurs in a forward (protecting) direction or a reverse direction.
Since a negative-phase-sequence circuit has no power supplies, it is in phase opposite to that of a positive-phase-sequence circuit. The phase of current leads that of voltage when a forward fault occurs and the former lags the latter when a reverse fault does. For example, the phase of current advances by 90 degrees to obtain an inner product between current and voltage. If the inner product is positive, it can be determined that a reverse fault has occurred. If it is negative, it can be determined that a forward fault has occurred. The following is the actual determination computing expression:V2m×J2m+V2(m−3)×J2(m−3)<0  (3)where J2m represents a value obtained by advancing the phase of I2m by 90 degrees. The operating range of the negative-phase-sequence directional relay can be shown in FIG. 18 if it actually includes some dead zones K.
The above principle is very true of a zero-phase-sequence circuit in a ground fault. A zero-phase-sequence component is simply used in the amount of electricity to determine the ground fault.
However, when a single electric power pylon carries multiple circuit causing zero-phase-sequence cyclic currents to flow among the wires, or while one of three phases is temporarily disconnected for example during a period of dead time of a single-phase reclosing relay, there are unbalanced components such as negative-phase-sequence components and zero-phase-sequence components though no fault actually occurs in a power system. If an operation for determining a fault is carried out using an amount of electricity of the unbalanced components in this case, there is possibility that the directional relay will decrease in sensitivity or mal-operation.
An object of the present invention is to provide a digital directional relay that is capable of correctly determining a direction of an unbalanced fault even though a power system contains unbalanced components such as negative-phase-sequence components and zero-phase-sequence components in a steady state.